1. Field of the Invention
The present description relates to the generation of random or pseudo-random numbers in an integrated circuit, and in particular, but not exclusively, smart card integrated circuits or integrated circuits equipped with a communication interface conforming to the USB (Universal Serial Bus) standard.
2. Description of the Related Art
Random or pseudo-random numbers are used by numerous functions such as encryption functions or protection functions for protection against attacks particularly by fault injection.
There are many methods for generating a random number. Thus, certain methods use an internal noise signal that is sampled to generate a random variable. The noise signals are generally of very low amplitudes. It is therefore necessary to amplify them. This amplification introduces a correlation factor, because the amplifier is also subjected to the same conditions (same technology, supply voltage, temperature, etc.) as the internal source of noise. Such a correlation enables the random variable generator to be controlled from the external environment, and therefore to be used to inject errors into the integrated circuit if it is used in a smart card.
In U.S. patent application Ser. No. 09/805,265, a first sawtooth signal is sampled by a second signal. The two signals are asynchronous. The amplitude of each sample is compared with a threshold voltage, and the result of the comparison supplies the value of a bit of a pseudo-random number. This method for generating a random number requires two sources of signals to be implemented. The random nature of the numbers generated depends, in particular, on the level of correlation between the source of the sawtooth signal and the source of the sampling signal. Now, in an integrated circuit, such as those used to equip smart cards, it is difficult to produce decorrelated oscillators, due to the fact that these units are produced with the same technology and are subjected to the same environmental conditions (temperature, sound and light) and electrical supply conditions (average amplitude, level of amplitude and frequency interference). In smart cards, the use of an external source of signals is not desirable because it may impair the resistance of the chip to attacks.